1. Field of the Invention
This invention relates to an intake system for an internal combustion engine, and more particularly to an intake system having a main intake passage which leads to each combustion chamber and is provided with a shutter valve to be opened when the amount of depression of the accelerator pedal exceeds a predetermined value and a bypass passage which is provided to bypass the shutter valve, is smaller than the main intake passage in cross section and is provided with a timing valve for closing the bypass passage at a predetermined time in each cycle.
2. Description of the Prior Art
In the intake system disclosed in Japanese unexamined Patent Publication No. 58(1983)-23245, an intake passage is provided with a shutter valve which is opened when the amount of depression of the accelerator pedal becomes not smaller than a predetermined value, and a bypass passage is provided to bypass the shutter valve. The bypass passage is smaller than the intake passage in cross section and is provided with a rotary valve which is rotated at half the rotational speed of the camshaft. When the amount of depression of the accelerator pedal is smaller than the predetermined value, that is, in the light load range, the shutter valve is closed so that intake air is introduced into the combustion chamber solely through the bypass passage, thereby increasing the flow speed of intake air to generate swirl of air-fuel mixture in the combustion chamber so as to improve combustion in the combustion chamber. The bypass passage is closed by the rotary valve in the middle of the intake stroke to reduce pumping loss, thereby improving fuel economy. When the amount of depression of the accelerator pedal is not smaller than the predetermined value, that is, in the heavy load range, the shutter valve is opened to open the intake passage and intake air is introduced into the combustion chamber through both the intake passage and the bypass passage, thereby increasing output power.
However, such an intake system gives rise to the following problem. In the light load range, reduction of pumping loss is obtained by a so-called Miller cycle generated by closing the rotary valve in the middle of the intake stroke. When the amount of depression of the accelerator pedal is increased to the predetermined value and the shutter valve is opened, intake air comes to be introduced also through the intake passage and the actual end of intake is abruptly changed from the middle of the intake stroke to the end of the intake stroke. That is, operation of the engine is abruptly changed from the Miller cycle to the normal or Otto cycle, causing torque shock, and accordingly the engine output power cannot be smoothly increased.